1. Technical Field
The present invention is related to a chain tensioner, particularly of the type used for controlling the operating tension in timing chains which operate valve gear in automotive internal combustion engines.
2. Prior Art
Chain and belt tensioners are known, for example, from U.S. Pat. No. 3,407,674; and German DEOS 23 33 688; DE-PS 908 694; DE-PS 40 01 303; DE-PS 40 01 304; DE-PS 22 12 818; DE-PS 25 25 352; or DE-OS 25 57 109. Commonly, a piston moving back and forth in a cylinder is preloaded by mechanical or hydraulic means in such a way that its free end protruding from the open end of the cylinder rests on a chain so as to produce a predetermined amount of tension. This should cause the chain to be under tension over its entire travel, in order to minimize wear and to damp vibration and noise.
Chain tensioners provided in internal combustion engines respond to longitudinal stretching of the chain and control longitudinal and transverse vibrations of the chain, which may be caused by excitations arising from the camshaft, the crankshaft and associated components. The damping behavior of known chain tensioners, which place the chain under spring load and hydraulic support, is generally dependent upon a leakage flow between the cylinder wall and the piston. High-frequency load impacts and the corresponding high-frequency chain vibrations are unsatisfactorily damped, which has a negative effect on the mechanical efficiency and service life of such dampers.
The present invention provides a chain tensioner which attenuates high-frequency vibrations by means of a pressure-loaded attenuation device interposed between the mechanical and hydraulic tensioning device and the closed end of the cylinder housing. A chain tensioner according to the present invention has a mechanical and hydraulic preloading device to which "impacts" coming from the chain are first imparted. The impacts are partially compensated or attenuated by the mechanical and hydraulic preloading device. Noncompensated impacts, especially high-frequency vibrations, are transmitted to the basic piston and then compensated via the attenuation element positioned in series with a basic piston. Thus, the invention deals with a series connection of two attenuation devices, whereby the first attenuation device facing the chain is supported by the parallel arrangement of an attenuation device and a mechanical preloading device (spring) and a hydraulic attenuation device, whereby the attenuation is designed such that those vibrations which are not compensated by the mechanical/hydraulic preloading device acting as parallel circuit, that is, in particular, high-frequency vibrations, upon which the hydraulic fluid acts as a rigid body, thereby transmitting the vibrations to the attenuation device provided according to the invention and damped there.
In a preferred embodiment of the invention, the previously described attenuation device may comprise an elastomeric damping element which is under pressure preload, and which has one end resting against the closed end of the cylinder and another end contacting a front end area of a basic piston placed in the cylinder, whereby the piston is supported via the mechanical/hydraulic preloading device against the other front end of the basic piston.
The preloading of the damping element can be triggered by pressing together the elastic material and fixing it by means of a preloading device. Preferably, the preloading device for the attenuation element has a preloading end collar with its outer periphery inside a groove in the cylinder interior wall and runs transversely to the cylinder longitudinal direction, and which holds the elastic damping element under pressure preload.